There is known a so-called noise canceling headphone which can reduce noise of the surroundings by collecting noise around the user by a microphone with a headphone incorporated into a housing, analyzing the noise, and outputting a negative phase sound from the signal-audio converter element (hereinafter called the “driver unit”) of the headphone.
In this case, as for the noise canceling system, two types are available: a feed-forward system and feedback system. FIG. 9 shows a configuration of the noise canceling headphone of the feed-forward system, while FIG. 10 shows a configuration of the noise canceling headphone of the feedback system.
In the feed-forward system of FIG. 9, the microphone element is arranged at such a location that a noise around the microphone element is collected but a sound released from the driver unit 2 is not collected. The electric signal from the microphone element 1 corresponding to the surrounding noise collected is supplied to the equalizer circuit 3.
This equalizer circuit 3 has a phase and amplitude characteristic, that is, a frequency characteristic, optimally designed in order to obtain an audio signal for canceling the surrounding noise inputted. An output signal of the equalizer circuit 3 is supplied to an AMP 5 via an adder circuit 4.
This AMP 5 is optimally designed so that the gain of the audio signal for canceling the sounding noise becomes optimal. The audio signal for canceling the surrounding noise from the AMP 5 is supplied to the driver unit 2. That is, the driver unit 2 functions as a sound source for canceling the surrounding noise of the user.
As described above, the sound of a phase negative to that of the surrounding noise is emitted in the vicinity of user's ears and is acoustically synthesized with the surrounding noise. As a result, the surrounding noise is canceled and the sound with reduced surrounding noise is listened to by the user.
In the case such as this, from an audio signal input terminal 6, music signals, etc. are supplied, added at the adder circuit 4, and supplied to the driver unit 2 via the AMP 5, and music is played back. At this time, the user can comfortably enjoy high-quality music without excessively turning up the volume even if the surrounding noise is large, because it is canceled and reduced as described above.
Next, in the feedback system of FIG. 10, the microphone element 1 collects the synthesized sound between the surrounding noise and the sound emitted from the driver unit 2 near the user's ears. The the frequency characteristics (phase and amplitude characteristics) of the equalizer circuit 3 are optimally designed in such a way that the synthesized sound collected by the microphone element 1 becomes below a predetermined level. The gain of AMP 5 is also designed to achieve optimum canceling effects on the surrounding sound.
In the feedback system, as shown in FIG. 10, the adder circuit 4 is mounted on the input side of the equalizer circuit 3. And the music signal, etc. is supplied from the audio signal input terminal 6, added at the adder circuit 4, and supplied to the driver unit 2 via the equalizer circuit 3 and the AMP 5, and the music is played back. In this event, the user can comfortably enjoy the high-quality music without excessively turning up the volume because the surrounding noise is canceled as described above even if it is noisy.
Because the noise canceling headphone has the above-mentioned advantages, it is utilized for listening to the music in the aircraft with engine noises reduced.
However, a conventional noise canceling headphone has a component element for noise canceling, which is structured to be integral with a headphone section in such a way that it is practically inseparable from the headphone section. The reason for the integral structure is that the noise canceling headphone creates the negative phase component of the noise signal over a broad band exceeding 1 decade and cancels the noise from the viewpoint of the operating principle, but because of large variations of audio characteristics as well as for absorbing the variations of individual noise canceling component elements, all the elements are integrated into one and comprehensively adjusted and corrected to maximize the canceling effects.
However, this kind of integral structure has following problems:    1) Even when part of the component elements, such as ear pads that come in contact with the human body, must be replaced for hygienic reasons, the whole must be replaced;    2) When part of the component elements is replaced, the whole must be adjusted again; and    3) Whether the whole or part is replaced, the maintenance expenses generated in the case become great.
It is an object of the present invention to provide an acoustic apparatus that can clear away the problems as mentioned above.